1. Field of the Art
The present invention relates to a local oscillation circuit which is usable as a tuner circuit in a television receiver, a CATV converter or an indoor unit of satellite receiver.
2. Background of the Prior Art
Recently, with the spread of CATV, multi channel receiving in television tuners and hence, an up-down type tuner or CATV converter is necessary. In one system, an input signal is converted to an intermediate frequency signal of higher frequency than the received signal and the intermediate frequency signal is then converted to a second intermediate frequency signal after passing through a narrow width band-pass filter (for example, published unexamined patent application SHO No. 59-149405). A feature of the system is that multi-channel receiving is achieved using simple structure by varying an oscillation frequency of a local oscillator in a range of over 1 GHz. The local oscillation circuit disclosed in published unexamined patent application SHO No. 59-149405 will now be described with reference to the Figures.
In FIG. 1, the local oscillation circuit is comprised of an oscillation amplifier 1, a resonance circuit 2, and can vary its oscillation frequency by an external tuning DC voltage impressed on a terminal 1A. The oscillation output signal is amplified in an amplifier 3 and fed to an external load circuit through an output terminal IB. The amplifier 3 operates as a buffer for the external load circuit and improves the local oscillation circuit.
FIG. 2 shows a circuit diagram of FIG. 1. Blocks encircled by broken lines correspond to blocks in FIG. 1. The amplifier 1 for oscillation is provided in a manner that the collector of an oscillation transistor Q.sub.1 is grounded with regard to alternating current through a by-pass capacitor to make a negative impedance seen from the base of the oscillation transistor Q.sub.1. Bias resistor R.sub.1 is connected between the collector and base of oscillation transistor Q.sub.1. The base of the oscillation transistor Q.sub.1 is connected through a coupling capacitor C.sub.2. A series resonant circuit is formed by a variable capacitance diode D.sub.1, a resonant inductance L.sub.1, and the internal capacitance of the transistor Q.sub.1. By grounding the other end of the series resonant circuit with regard to alternating current, negative gain seen from the base of transistor Q.sub.1 and loss of resonant inductance L.sub.1 are offset, and it oscillates with such a frequency that the phase characteristics seen from the base of transistor Q.sub.1 become 2n.pi.(n=0,1,2,3 . . . )rad.
By varying a tuning D.C. voltage impressed on a terminal 1A, the capacitance of the variable capacitance diode D.sub.1 varies, and amplitude and phase characteristics vary. The tuning voltage passes through bias resistor R.sub.4. By-pass capacitor C.sub.6 is grounded from the terminal 1A. The oscillation output signal is obtained at a common connection point between the emitter of transistor Q.sub.1 and a choke coil L.sub.2 by grounding the emitter of transistor Q.sub.1 with regard to direct current through a resistor R.sub.3 with connected in series choke coil L.sub.2 to the emitter of transistor Q.sub.1. The oscillation output signal is fed to the base of an amplifier transistor Q.sub.2 through a coupling capacitor C.sub.3. A potential is applied to the base of transistor Q.sub.2 from a source voltage Vcc impressed on a terminal 1D through a choke coil L.sub.3 and resistor R.sub.6. The emitter of amplifier transistor Q.sub.2 is grounded and the collector of transistor Q.sub.2 is connected to the terminal 1D through the choke coil L.sub.3. The amplified oscillation output signal is fed to the exterior load from the collector of transistor Q.sub.2 through a coupling capacitor C.sub.5 and an output terminal 1B.
Two problems exist in the prior art of FIGS. 1 and 2. The first one is that when the tuning potential impressed on the terminal 1A is varied, a current flowing in the oscillation transistor Q.sub.1 varies a great amount with changes in frequency, for example, from 15 mA to 35 mA. This variation is not suitable for use with the oscillation transistor, and the current in some cases happens to go over an allowable collector dissipation PC. A second shortcoming is that because of the structure of amplifier 3 shown in FIG. 2, a current of around 15 mA becomes necessary, and electric power consumption in the whole circuit increases.
Also, in the circuit shown in FIG. 2, the variable frequency range of the oscillator is limited. This is because the resonance capacitance is a series combination of the parallel sum of the capacitances of the variable capacitance diode, the stray capacitance of the circuit, and the input capacitance (mainly reverse bias capacitance Cob) of oscillation transistor Q.sub.1. The variable frequency range of the oscillator is limited mainly by the reverse bias capacitance Cob of oscillation transistor Q.sub.1.
The circuit shown in FIGS. 3 and 4 attempts to increase the frequency range. The base voltage of oscillation transistor Q.sub.1 is changed by inputting an external change-over d.c. voltage to a terminal 1E through a resistor R.sub.7. When a voltage of the terminal 1E becomes high, the base voltage of oscillation transistor Q.sub.1 rises, and the collector current of oscillation transistor Q.sub.1 increases. The reverse bias capacitance Cob of the oscillation transistor increases correspondingly to the increase of the collector current. The collector of transistor Q.sub.1 is also fed with a current from a power source Vcc at terminal 1C through a resistor R.sub.2. The collector potential of oscillation transistor Q.sub.1 drops with the increase of the collector current, and the reverse bias voltage between the base and the collector becomes smaller, and therefore causes the capacitance Cob to increase. Because the capacitance inserted in series against the resonance circuit 2 increases with increasing reverse bias capacitance Cob, the oscillation frequency as a whole shifts toward a lower frequency. This is advantageous because the variable range of frequency at the low frequency end can be extended, in contrast with the circuit of FIGS. 1 and 2 where this is not the case.
However, even in the prior art shown in FIGS. 3 and 4, there are similar problems as exist in the circuit shown in FIGS. 1 and 2. First, the current flowing in the oscillation transistor Q.sub.1 increases a great amount, for example, from 20 mA to 40 mA, when the change-over potential impressed on the terminal 1E is made high, and it often exceeds the allowable collector dissipating PC of the oscillation transistor. The second shortcoming is that electric power consumed in the whole circuit increases in a way similar to that of the circuits in FIG. 1 and FIG. 2.